Method of detecting a fluid surface

ABSTRACT

A sensor for detecting contact of a fluid delivery probe with a fluid surface and for detecting fluid flow through the probe includes a first electrode disposed along a fluid flow path of the probe upstream from a distal tip of the probe and a second electrode longitudinally spaced and electrically isolated from the first electrode and disposed at the distal tip of the probe. An oscillating signal is transmitted through the first electrode, and at least a portion of the signal is received through the second electrode. Through changes in the received signal due to the distal tip of the probe coming into contact with a fluid surface or due to fluid flow through the conduit between the first and second electrodes, fluid surface contact and fluid flow can be detected. A pressure sensor can be employed to monitor internal fluid pressure within the fluid conduit of the fluid delivery probe as a secondary, redundant mechanism for detecting fluid flow through the conduit.

This application is a divisional of U.S. patent application Ser. No.09/794,255, filed Feb. 28, 2001 now U.S. Pat. No. 6,604,054, whichclaims the benefit of U.S. Provisional Application No. 60/185,741, filed29 Feb. 2000, the entire disclosure of which is incorporated byreference.

BACKGROUND OF THE INVENTION

This invention relates to a system and method for verifying the movementof an amount of fluid through a fluid delivery probe and/or fordetecting a fluid surface within a container that is entered by thefluid delivery probe.

Automated analyzers are commonly used by clinical laboratories and inhealth science research to assay and determine inter alia the presenceor amount of a particular analyte or group of analytes in a biologicalsample. Typical biological samples for assaying include blood, urine,cerebrospinal fluid, pus, seminal fluid, sputum, stool, plants, waterand soil. Analytes commonly targeted in biological samples includeantibodies, antigens, nucleic acids, toxins and other chemicals.Clinicians especially prefer automated analyzers over manual proceduresbecause of their high-throughput capabilities, reduced labor expenses,and the limits they place on human error that can lead to false ormisleading results. To be most useful, an analyzer preferably automatesboth the sample preparation and sample processing steps of an assay.

Sample preparation may be initiated by an automated fluid transfersystem which transfers a fluid sample from a sample container to areaction vessel for analysis. The automated fluid transfer system mayalso be used to transfer one or more assay reagents from theirrespective containers or associated reservoirs into the sample-holdingreaction vessel. After conducting the appropriate sample processingsteps for a given assay, the contents of the reaction vessel may beexamined by the automated analyzer to determine the presence or amountof at least one specifically targeted analyte. Detecting a targetedanalyte in the sample might provide an indication that a particularpathogenic organism is present in the sample, or it might indicate aspecific disease condition or state useful for determining or adapting atreatment regimen.

The fluid transfer system typically includes a fluid delivery probeoperatively carried on a robotically controlled arm to performaspiration and dispensing functions required for the transfer processand a pump coupled to the probe by a conduit system. During a fluidtransfer operation, the robotic arm, under the command of a systemcontroller, positions the fluid delivery probe above a sample or reagentcontainer and moves the probe into the container until the tip of theprobe reaches the fluid surface in the container. It is desirable thatthe distal tip of the probe be maintained right at the fluid surface toavoid ingesting air into the probe during aspiration and to avoidpossible cross-contamination that can occur if the probe isunnecessarily submerged into the fluid and fluid residue is carried onthe exterior of the probe from one sample to another. Accordingly, adesirable feature of an automated fluid delivery probe is a means bywhich contact of the probe tip with the fluid surface can be detected asthe probe is being lowered into a fluid-containing vessel.

With the probe tip maintained at the fluid surface, a pump, such as asyringe type pump, is activated to draw an amount of sample or reagentfluid from the container into the probe. The amount of fluid aspiratedwill correspond to the volume and number of aliquots to be dispensedfrom the probe. The fluid delivery probe is thereafter moved into aposition above a reaction vessel and a precise aliquot of fluid isdispensed. To ensure that accurate results are obtained in the tests, apredetermined volume of the sample must be accurately aspirated anddispensed into the reaction vessel. Accordingly, another desirablefeature of an automated fluid delivery probe is automated verificationof fluid dispensed from the probe.

Different devices and methods for automatically determining when a probetip has contacted a fluid surface in a container have been proposed inthe available literature. For example, some surface detection sensorsoperate on the basis of capacitance. The probe, if made from aconductive, e.g., metal, conduit, will exhibit a finite amount ofelectrical capacitance. When the probe tip contacts a fluid surface, thehigher dielectric constant and greater surface area of the fluid resultsin a small, but measurable, increase in the capacitance of the probe.

Other surface detection mechanisms for incorporation onto a fluiddelivery probe include two or more electrodes which may comprise tubularelements arranged coaxially with each other (see, e.g., U.S. Pat. Nos.5,304,347 and 5,550,059) or elongated conductors extending along thelength of the probe and arranged in a spaced, parallel relationship(see, e.g., U.S. Pat. Nos. 5,045,286 and 5,843,378). When the probecontacts a fluid surface, the fluid, which contacts both electrodessimultaneously, electrically couples the electrodes to each other. If avoltage is applied across the electrodes the electrical coupling causedby the electrodes contacting the fluid surface results in a measurablechange in the voltage drop across the electrodes.

U.S. Pat. Nos. 5,013,529 and 5,665,601 describe surface detectiondevices which incorporate a pressure sensor connected to a fluid linethrough which constant pressure gas is expelled through the tip of theprobe. When the tip contacts the fluid surface, thereby blocking the gasemitting orifice (i.e., the end opening of the probe), a measurablechange in the pressure is exhibited. U.S. Pat. No. 6,100,094 describes asurface detection device which includes an optic emitter which emitslight axially through, or alongside, a tip. The light is reflected fromthe fluid surface back into the tip to a light sensor disposed withinthe tip. The amount of light reflected back to the light sensordetectably changes when the tip contacts the fluid surface.

The prior art surface detection sensors described above each suffer fromcertain shortcomings. For example, achieving adequate accuracy andrepeatability with capacitive surface sensors can be difficult becausethe change in capacitance exhibited when a probe contacts a fluidsurface can be very small and thus difficult to detect. This isespecially true where the fluid is a conductive fluid with a lowdielectric value. Furthermore, because of the small capacitance changesexhibited, capacitive surface detection sensors can be susceptible toinaccuracies due to fluctuating stray capacitances caused by adjacentmoving structures or changes in the amount of fluid contained in theprobe and/or container.

Dual electrode surface detection devices constructed to date, withside-by-side or coaxial arrangement of the electrodes, are complex andcumbersome. Surface detection devices that emit constant pressure gascan cause disturbances and even bubbling and/or atomization of thefluid. The effectiveness of optic sensors can be diminished due toresidue or other buildup on the optic emitter and/or receiver.

Other devices and methods are described in the available literature forverifying aspiration and/or delivery of a fluid from the probe. Forexample, U.S. Pat. No. 6,121,049 describes a system wherein the pressureneeded to hold up a column of aspirated fluid in the probe can bemeasured and compared to a predetermined standard to determine if aproper amount of fluid has been aspirated. By verifying a properaspiration, a proper subsequent fluid delivery can, theoretically, beinferred. U.S. Pat. No. 5,559,339 describes a system which includesoptical sensors, each with an emitter-receiver pair, disposed adjacentthe pipette tip. Fluid flowing from the tip breaks the electromagneticbeam between the emitter and receiver, thereby indicating the flow offluid. The duration of fluid flow can be monitored to determine if aproper amount of fluid has been dispensed.

Such fluid flow verification devices suffer from shortcomings which canlimit their effectiveness. Pressure sensors that measure the amount ofpressure required to hold up a column of aspirated fluid may beeffective for confirming a proper aspiration of fluid, but, becausefluid delivery can be interrupted by system leaks or occlusions blockingthe probe, such sensors do not necessarily provide confirmation ofproper fluid delivery. Furthermore, such devices are useful only forfluid delivery procedures that involve aspiration of fluid into theprobe prior to delivery of the fluid from the probe into a reactionvessel. Such devices will not provide confirming information for fluidtransfer systems in which fluid is pumped directly from a reservoirthrough the fluid delivery probe and into a reaction vessel withoutfirst being aspirated from another container.

As with surface detection devices that employ optic emitters andreceivers, the effectiveness of the optic sensors employed to verifyfluid flow can be diminished by residual build-up or other debrisinterfering with the emission or reception of the electromagnetic beam.

Accordingly the devices and methods described heretofore in the priorart are susceptible to further improvement. Moreover, although surfacedetection and fluid delivery verification are important features of aconsistently accurate automated fluid delivery probe, the prior art doesnot describe a simple, effective, and accurate method and device forproviding the combined capabilities of surface detection and fluiddelivery verification in a single fluid delivery probe. Finally, theprior art does not describe a fluid delivery verification method ordevice in which secondary, redundant means are employed for verifyingfluid delivery to guard against erroneous indications of proper fluiddelivery.

SUMMARY OF THE INVENTION

The present invention overcomes the shortcomings of and is animprovement over surface detection and fluid delivery verificationapparatuses described above.

In particular, the present invention comprises a sensor mechanism thatincludes a pair of longitudinally spaced, electrically isolatedelectrodes forming portions of a fluid flow conduit of a fluid deliveryprobe. The first electrode is disposed along a portion of the fluiddelivery probe upstream from the tip, and the second electrode isdisposed at the tip of the probe. An oscillating signal is transmittedby the first electrode, which functions as a transmitting antenna, andsome portion of the transmitted signal is received by the secondelectrode, which functions as a receiving antenna. The characteristicsof the signal received by the second electrode, i.e., the amplitudeand/or the phase difference of the signal, will change when the tip ofthe fluid delivery probe contacts a fluid surface and/or if there isfluid flow through the conduit between the first and second electrodes.By monitoring the received signal, the sensor, along with its associatedinterface circuitry, can provide both surface detection and fluiddelivery verification. Depending on the characteristics of the fluid,i.e., whether the fluid is an ionic or non-ionic fluid, the amplitude orthe phase of the received signal may exhibit a more pronounced change.In any event, the sensor is effective for surface detection and fluiddelivery verification for any type of fluid.

The sensor can be enhanced by incorporating a pressure sensor formonitoring internal system pressure during fluid delivery. Bydetermining whether a pressure signal profile obtained during anintended fluid delivery compares favorably with the profile that wouldbe expected for proper delivery of a particular fluid, the fluiddelivery can be verified. Thus, the pressure sensor provides asecondary, redundant verification to compliment the fluid deliveryverification provided by monitoring the signal received by the secondelectrode.

In a preferred manner of verifying a proper fluid delivery, theamplitude of the signal received by the second electrode is monitored orthe phase difference between the transmitted and received signals ismonitored (the amplitude and phase difference signals will begenerically referred to as the “tip signal”) during an intended fluiddelivery. In particular, the tip signal is integrated from a timeapproximating the intended initiation of fluid delivery to a timeapproximating the intended termination of fluid delivery. In additionthe tip signal variability is analyzed from the initiation time to thetermination time. The tip integral and the tip signal variability arecompared to accepted values experimentally determined for properdelivery of the particular fluid being delivered, and, if they are notwithin acceptable limits, an error signal is generated.

The tip signal is indicative of the continuity of fluid flow between thefirst and second electrodes. An irregularity in the tip signal, which isindicative of a discontinuity in fluid flow between the electrodes (dueto, e.g., pump malfunction, probe blockage, air bubbles in the dispensedor aspirated fluid, insufficient fluid available for dispensing), willresult in a tip signal integral and/or tip signal variability that isnot within accepted limits. On the other hand, a tip signal integral andtip signal variability that are within accepted limits are indicative ofa regular tip signal over the duration of the intended fluid deliveryand thus are indicative of a proper fluid delivery.

Similarly, a pressure signal is also obtained and analyzed to verify aproper fluid delivery. In particular, the initiation of a fluid deliverywill result in a detectable jump in the pressure signal from a steadystate, quiescent value, and termination of fluid delivery will result ina detectable drop in pressure toward the steady state value. The jumpand drop in the fluid pressure signal are located and the elapsed timebetween the jump and drop, termed the pulse width, is determined. Inaddition, the pressure signal is integrated over the pulse width. Thepressure integral and the pulse width are compared to accepted valuesexperimentally determined for proper delivery of the particular fluidbeing delivered, and, if they are not within acceptable limits, an errorsignal is generated.

The pressure signal reflects the continuity of the pressure level duringan intended fluid delivery. An irregularity in the pressure signal (dueto, e.g., pump malfunction, probe blockage, air bubbles in the dispensedor aspirated fluid, insufficient fluid available for dispensing), willresult in a pressure signal integral and/or pulse width that is notwithin accepted limits. On the other hand, a pressure signal integraland pulse width that are within accepted limits are indicative of aregular pressure signal of proper duration during the intended fluiddelivery and thus are indicative of a proper fluid delivery.Accordingly, the pressure sensor provides a secondary fluid deliveryverification to compliment the fluid delivery verification provided bythe first and second electrodes.

Having two electrodes, longitudinally spaced from each other and formingportions of the fluid delivery probe conduit, the sensor of the presentinvention is simple in construction and unobtrusive and adds little tothe overall size of the fluid delivery probe. Moreover, the sensor doesnot suffer from the deficiencies encountered with prior art sensorsdescribed above. In particular, the sensor of the present invention isnot sensitive to stray system capacitance, is effective regardless ofthe ionic properties of the fluid, does not rely upon potentiallyunreliable optic sensors, and does not emit a gas pressure stream thatcan disturb the fluid to be aspirated.

Other objects, features, and characteristics of the present invention,including the methods of operation and the function and interrelation ofthe elements of structure, will become more apparent upon considerationof the following description and the appended claims, with reference tothe accompanying drawings, all of which form a part of this disclosure,wherein like reference numerals designate corresponding parts in thevarious figures.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a robotic substance transfer mechanism;

FIG. 2 is a schematic view of a fluid delivery system embodied within asubstance transfer mechanism;

FIG. 3 is a perspective view of a fluid delivery probe incorporating afluid dispense and fluid surface verification device according to thepresent invention;

FIG. 4 is a partial side elevation of an upper portion of the fluiddelivery probe;

FIG. 5 is a partial transverse cross-section of the fluid delivery probealong the line V—V in FIG. 3;

FIG. 6 is a longitudinal cross-section of a ribbon cable assembly usedin conjunction with a sensor assembly that is part of the dispense andsurface verification device;

FIG. 7 is a transverse cross-section of the sensor assembly of the fluiddelivery probe;

FIG. 8 is a partial transverse cross-section of the sensor assemblyshowing the ribbon cable assembly connected to the sensor assembly;

FIG. 9 is a transverse cross-section of an alternate embodiment of thesensor assembly of the fluid delivery probe;

FIG. 10 is a block diagram illustrating the electrical sensing anddetection circuitry in the dispense and surface verification system;

FIG. 11 is a detailed block diagram of a dispense and surfaceverification interface circuit;

FIG. 12 is a circuit diagram of a phase detector circuit of theinterface circuitry of the dispense and surface verification system;

FIG. 13 is a circuit diagram of an auto-tune circuit of the interfacecircuitry;

FIG. 14 shows plots of a typical pressure time signal and a typicalsensor assembly time signal generated by the dispense and surfaceverification system; and

FIG. 15 shows plots of pressure-time signals as affected by varyingamounts of air entrained in fluid moving through the fluid deliverysystem.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A robotic substance transfer mechanism with which a fluid dispense andfluid surface verification system according to the present invention canbe operationally combined is generally designated by reference number 20in FIG. 1. The robotic substance transfer mechanism 20 into which thedispense and surface verification system of the present invention can beincorporated may be an off-the-shelf device, such as a Model No. RSP9000 Robotic Sample Processor available from Cavro Inc. of Sunnyvale,Calif. On the other hand, while the dispense and surface verificationsystem of the present invention is described herein primarily in thecontext of its incorporation into a robotic substance transfermechanism, such as that shown in FIG. 1, the system can as well beincorporated into any mechanism which performs an automated fluiddelivery function and in which fluid dispense verification and/or fluidsurface detection is required or advantageous.

The robotic substance transfer mechanism 20 includes a fluid deliveryprobe 50 having a fluid delivery conduit assembly 52 and mounted on agantry assembly to provide X, Y, and Z motion. In particular, the fluiddelivery probe 50 is mounted on a longitudinal translation boom 24, andthe longitudinal translation boom 24 is mounted on and supported by alateral translation boom 22. X-Y motion in a horizontal plane can beeffected by motors disposed within a housing 26 for moving the fluiddelivery probe 50 along the longitudinal translation boom 24 and thelateral translation boom 22. In the illustrated embodiment, atranslation motor (not shown) within the housing 26 powers a drivingdevice that cooperates with a track 28 formed along the lateraltranslation boom 22 to move the housing 26 and the longitudinaltranslation boom 24 reciprocally along the lateral translation boom 22.Movement of the fluid delivery probe 50 along the longitudinaltranslation boom 24 may be effected by means of a motor (not shown)housed in the housing 26 and coupled to, for example, an endless beltdisposed within the longitudinal translation boom 24 and attached to thefluid delivery probe 50 or a lead screw threadedly coupled to the fluiddelivery probe 50 for moving the probe axially along the screw as thescrew rotates about its own axis. Another motor (not shown) is carriedon the substance transfer mechanism 20 along the longitudinaltranslation boom 24 and is coupled to the fluid delivery probe 50, forexample, by a lead screw or a rack and pinion arrangement, for effectingZ-axis, vertical movement of the fluid delivery probe 50.

The fluid delivery conduit assembly 52 extends into a tube protectorblock 54 disposed below the longitudinal translation boom 24. A rigidtube extension 34, preferably made from stainless steel tubing, extendsupwardly through a pipette slot 30 formed in the longitudinaltranslation boom 24, terminating at a position above the longitudinaltranslation boom 24.

Fluid delivery is performed by a pump 36, which forces fluid flowthrough a flexible tube 32, preferably made from polytetrafluoroethylene(PTFE), and into the rigid tube extension 34 and the fluid deliveryconduit assembly 52. The flexible tube 32, rigid extension 34 and thefluid delivery conduit assembly 52 together form at least a portion of afluid conduit system 33 through which the pump 36 moves fluid dispensedby the fluid delivery probe 50. In particular, pump 36 is preferably asyringe pump, such as a Cavro Model Number XL 3000 Modular Digital Pump.Other types of pumps may be used as well. Pump 36 may be coupled to anoptional, multi-port (preferably three-port) rotary valve 38. Theflexible tube 32 is connected to an output port of the valve 38 (ordirectly to the pump 36 if no valve is employed) and extends to and isconnected at the proximal end of the rigid tube extension 34 (see FIGS.1 and 2). In the exemplary embodiment shown in the figures, fluiddelivery line 37 carries fluid from a fluid reservoir or container,generally represented at 35, to the valve 38. A multi-port rotary valveallows the pump to be switched from the reservoir 35, from which fluidmay be drawn into the fluid delivery system by pump 36, to the fluiddelivery probe 50, thereby allowing fluid in the fluid delivery systemto be delivered (i.e., dispensed) by the pump 36 through the fluiddelivery probe 50. A multi-port rotary valve allows multiple fluidreservoirs and/or multiple fluid delivery probes to be alternatelycoupled to one another via a pump.

Fluid may also be drawn into the fluid conduit system 33 by the pump 36directly through the fluid delivery conduit assembly 52 operativelypositioned in a container of fluid. Proper positioning of the fluiddelivery conduit assembly 52 is facilitated by the surface detectioncapability of the dispense and surface verification system, as will bedescribed hereinbelow.

The dispense and surface verification system of the present inventionincludes an in-line pressure sensor 40 located along the flexible tube32 between the pump 36 and the fluid delivery probe 50. Pressure sensor40 detects when a fluid (including a pure liquid or a solution, mixture,slurry, suspension, etc.) is moved by the pump 36 along the portion ofthe fluid conduit system 33 defined by the fluid delivery conduitassembly 52, the rigid tube extension 34, and the flexible tube 32. Inparticular, sensor 40 is able to differentiate resistance to fluid flowbased on fluid composition. Thus, the pressure indicated by sensor 40would be detectably different for a liquid moved through the conduitthan for air moved through the conduit. A preferred sensor is aHoneywell model 26PCBFG5G flow-through pressure sensor because it is aself-calibrating sensor that compensates for changes in ambienttemperature and because it is a robust device with silicone sealingwhich protects electronic strain gauges attached to a pressure-sensitivediaphragm located inside the sensor. The function and operation of thepressure sensor 40 will be described in further detail below.

The fluid delivery probe 50 will now be described with reference toFIGS. 3-5. The tube protector 54 is comprised of an upper portion 56,shown in the drawings as having the general shape of a rectangularsolid, and a lower portion 58, having a generally cylindrical shape asshown in the drawings. A through-hole 57 is formed through the upperportion 56, and a through-hole 59 is formed through the lower portion58. The aligned through-holes 57 and 59 receive a transfer tube 102 ofthe fluid delivery assembly 52 with a sliding fit between the tube 102and the through-holes 57 and 59. The upper portion 56 and the lowerportion 58 of the tube protector 54 are preferably formed from apolymeric material and most preferably from an injection moldedthermoplastic, such as Lexan®.

A cable connector housing 62 is attached at one portion thereof to thetube protector 54 and at another portion thereof to the transfer tube102. The cable connector housing 62 includes an upper portion 76, anangled portion 78, and a tube connecting portion 80. The cable connectorhousing 62 is also preferably formed from an injection moldedthermoplastic, such as Lexan®. An extruded aluminum bracket 42 forms apart of the cable connector housing 62 by an insert molding process. Aflange 48 of the bracket 42 projects from the cable connector housing 62and is attached to the tube protector 54 by means of one or morefasteners 44 extending through openings formed in the flange 48 and intothe tube protector 54.

A cylindrical opening 82 is formed in the tube connecting portion 80. Abottom end 84 of the tube connecting portion 80 has a through-hole 86formed therein and thereby provides a partial closure of the cylindricalopening 82. A stop element 53 is secured to the transfer tube 102 at anintermediate position along its length. In the preferred embodiment,both the stop element 53 and the transfer tube 102 are made fromstainless steel, and the stop element 53 is secured to the transfer tube102 by brazing. The dimensions of the transfer tube 102 (i.e., length,inside diameter, and outside diameter) will depend on the application.The cable connector housing 62 is attached to the transfer tube 102 byinserting the transfer tube 102 through the through-hole 86 until thestop element 53 is received within the opening 82, which is sized andshaped so as to conform to the stop element 53. The diameter of thethrough-hole 86 is smaller than the inside diameter of the cylindricalopening 82 and the outside diameter of the stop element 53. Thereforethe stop element 53 bottoms out at the bottom end 84 of the tubeconnecting portion 80. The tube connecting portion 80 is secured to thestop element 53 and the transfer tube 102 by means of epoxy which fillsthe opening 82. A preferred epoxy is available from Master Bond, Inc. ofHackensack, N.J., product number EP 42HT.

In the illustrated embodiment, a plastic spacer element 60 is disposedbetween the lower portion 58 of the tube protector 54 and the tubeconnecting portion 80 of the cable connector housing 62. The lowerportion 58 rests against the spacer element 60, and the spacer element60 fills a gap created between the top of the tube connecting portion 80and the bottom end of the lower portion 58 when the bottom end of thelower portion 58 contacts a top part of the angled portion 78, therebyblocking the lower portion 58 from contacting the top end of the tubeconnecting portion 80. Of course, if the geometries of the lower portion58 and the tube connecting portion 80 are such that the lower portion 58can rest directly on the tube connecting portion 80, without beingblocked by the angled portion 78, the spacer element 60 may be omitted.

A blind opening 73 is formed in an upper portion 76 of the cableconnector housing 62. In the preferred embodiment shown in the figures,a cylindrical section 46 of the bracket 42 forms the sides of theopening 73. A coaxial cable connector 68 is attached to an upper end ofthe upper portion 76 at the mouth of the opening 73, preferably byinserting a lower end 69 of the connector 68 into an upper end of thecylindrical section 46. A suitable cable connector is available fromLemo, Inc. of Santa Rosa, Calif., model number ERA 0125DLL. An externalcoaxial cable 64 can be attached to the connector 68 for transmittingsignals to the interface circuitry described below.

A coaxial ribbon cable 66 is electrically connected to the cableconnector 68 by means of a ribbon connector interface 70 which iscrimped onto an exposed end of the ribbon cable 66. FIG. 6 shows alongitudinal cross section of the ribbon cable 66. As shown in FIG. 6,the ribbon cable 66 comprises a multi-layer structure having at itscenter an electrically conductive core 90 running the entire length ofthe cable 66. Core 90 is preferably a copper strip having a preferredthickness of 0.003 inches and a preferred width of 0.03 inches. An innerinsulation layer 91, preferably polyester, surrounds the core 90. Asilver shielding layer 92 is sprayed onto the inner insulation layer 91so as to completely surround the core 90 and the inner insulation layer91. An outer insulation layer 93 of a Teflon®-type material is sprayedonto the silver layer 92 so as to completely surround the core 90, theinner insulation layer 91, and the silver shielding layer 92.

At one end of the cable 66 (the right end as shown in the figure), theinner and outer insulation layer 91, 93 and the silver shielding layer92 are removed from the core 90 so as to present an exposed section 94of the core 90. Exposed section 94 is attached to the connector 68 viathe ribbon connector interface 70.

At the opposite end of the cable 66 (the left side as shown in thefigure), the outer insulation layer 93, the silver shielding layer 92,and one half of the inner insulation layer 91 are removed from the cable66 so as to present an exposed section 99 of the core 90 with a portion98 of the inner insulation layer 91 bonded to one side thereof. To theimmediate right of the exposed sections 98 and 99, portions of thesilver shielding layer 92 and the outer insulation layer 93 are removedfrom the cable to form exposed section 97 of the inner insulation layer91. To the immediate right of the exposed section 97, an exposed section96 of the silver shielding layer 92 has the outer insulation layer 93removed therefrom.

As can be appreciated, the layers at the opposite ends of the cable 66are made into a tiered formation. The purpose of this tiered formationwill be explained below.

The ribbon cable 66 is preferably insert molded into a lower end of theupper portion 76 of the cable connector housing 62 and thereafterextends into the opening 73. The cable connector 68 and the portion ofthe ribbon cable 66 extending into the opening 73 are secured to thecable connector housing 62 by means of epoxy filling the opening 73. Inparticular, the opening 73 is filled with a lower epoxy layer 74,preferably comprising Master Bond EP-21TDC/S silver epoxy, and an upperepoxy layer 72, preferably comprising Master Bond EP-30 epoxy. Twodifferent types of epoxy are used to secure the ribbon cable 66, becausethe different epoxies react differently with the exposed and non-exposedsections of the cable 66. Master Bond EP-21TDC/S silver epoxy is used inthe lower epoxy layer 74 because this type of epoxy is caustic and woulddamage the exposed portion 94 of the core 90 near the connectorinterface 70. On the other hand, the EP-30 epoxy used in the upper epoxylayer 72 is not caustic to the exposed portion 94, but will not adhereto exposed section 95 of the insulating protective layer 91. TheEP-21TDC/S epoxy will adhere to the insulating protective layer 91 andthereby secure the covered portion of the ribbon cable 66 within theopening 73. The layer 91 on the ribbon cable 66 protects the core 90 ofthe cable 66 from the caustic effects of the EP-21TDC/S epoxy.

Alternatively, the cable 66 can be insert molded within the upperportion 76 of the cable connector housing 62 so that substantially onlythe exposed end 94 thereof extends into an opening in the upper portion76 that is shorter in length than opening 73. Thus, the lower epoxylayer 74 can be eliminated and the cable can be set within the housing62 by a single layer of non-caustic epoxy, such as Master Bond EP-30epoxy.

The details of the fluid delivery conduit assembly 52 will be describedwith reference to FIG. 7. The assembly 52 includes the transfer tube 102extending down from the rigid tube extension 34 and through the tubeprotector 54 and the tube connecting portion 80 of the cable connectorhousing 62. As indicated above, the transfer tube 102 is preferablyformed of stainless steel and includes a tapered tip 104 at a distal endthereof.

A sensor assembly 100 is arranged at the distal end of the transfer tube102. The sensor assembly 100 includes an isolating sleeve 112 having oneend thereof inserted over the tapered tip 104 of the transfer tube 102.The isolating sleeve 112 preferably comprises a tube constructed ofpolyethylene terephthalate (PET). Another suitable material for theisolating sleeve 112 is polytetrafluoroethylene (PTFE), although PTFE isless desirable than PET because it has been determined that proteindeposits can form on PTFE, and these deposits are slightly conductive. Atip element 106 is inserted into an opposite end of the isolating sleeve112 so that it is axially spaced from the distal end of the transfertube 102. Tip element 106 is preferably a stainless steel tube having avariable outside diameter defining an upper section 110 and a lowersection 108, whereby the upper section 110 has a greater outsidediameter than the lower section 108. The size of the upper section 110conforms to the size of commercially available material employed for theconstruction of the isolating sleeve 112. The lower section 108 was madeto have a smaller inner and outer diameter in accordance with the sizeof the opening of a vessel into which the fluid delivery conduitassembly 52 is to deliver fluid. It is not necessary to the operation ofthe sensor assembly 100, however, that the tip 106 have two sections ofdifferent inner and/or outer diameters.

The isolating sleeve 112 is secured to the transfer tube 102 and the tipelement 106 by means of epoxy, preferably Master Bond EP-42HT epoxy.

A tip interface element 114 is secured to a lower end of the isolatingsleeve 112. The tip interface element 114, preferably formed ofstainless steel, includes an upper, generally cylindrical section 115having an inside diameter sized so as to snugly fit over the outersurface of the lower portion of the isolating sleeve 112, and a narrowneck section 116 at a lower end thereof having an inside diameter sizedso as to snugly fit over the upper section 110 of the tip element 106.The inner surface of the cylindrical section 115 of the tip interfaceelement 114 is secured to the outside of the isolating sleeve 112 bymeans of epoxy, preferably Master Bond EP-42HT epoxy. The neck section116 is secured to the tip element 106 by means of a laser micro-weld.The coaxial ribbon cable 66 extends downwardly from the cable connectorhousing 62 along the outside of the transfer tube 102 and the isolatingsleeve 112 and an exposed section 118 of the cable 66 is attached to thetip interface element 114. A cover sleeve 120 covers the sensor assembly102, as will be described below.

FIG. 8 shows an enlarged view of a longitudinal cross-section of thelower end of the sensor assembly 100 illustrating the preferred mannerin which the coaxial ribbon cable 66 is attached to the assembly 100.For clarity, the cover sleeve 120 is not shown in FIG. 8.

As shown in FIG. 8, the exposed section 99 of the cable 66 is solderedto the upper section 115 of the tip interface element 114. The exposedsection 98 of the insulation layer 91 on one side of the exposed section99 opposite the side soldered to the tip interface element 114 minimizesnoise (i.e., stray, unwanted electrical emissions, emi, emf) picked upby the core 90 and also provides a protective layer between the coversleeve 120 (not shown in FIG. 8) and the exposed section 99. The shortsection 97 of the layer 91 provides a separation between the tipinterface element 114 and section 96 of the silver shielding layer 92 tofurther limit noise within the cable 66 by preventing contact betweentip interface element 114 and the silver shielding layer 92. The tieredconfiguration of the cable 66 formed by the exposed section 96 of thesilver shielding layer 92 provides a less drastic transition between thethin end of the cable at exposed sections 98 and 99 and the fullthickness of the cable 66 attached to the side of the transfer tube 102,thereby providing a relatively gradual transition to be covered by thesleeve 120. This makes it easier to fit the sleeve 120 over the assembly100 and also eliminates drastic discontinuities in the thickness of theassembly 100 which can cause tears in the sleeve 120. The silvershielding layer 92 is grounded to the transfer tube 102 by exposing aportion of the silver shielding layer 92 and connecting the exposedportion to the transfer tube by silver solder or conductive silverepoxy, generally indicated at 101 in FIG. 8.

The tip element 106 is preferably coated, inside and out, with anon-stick material, such as Teflon®, available from E. I. du Pont deNemours and Company. The purpose of the non-stick coating is to minimizehanging fluid drops clinging to the end of the tip element 106 and alsoto facilitate tip cleaning between fluid transfers.

The cover sleeve 120 covers and protects the sensor assembly 100 and theupper portions of the transfer tube 102 between the sensor assembly 100and the tube protector 54 and further covers and protects the coaxialribbon cable 66. The cover sleeve is preferably a resilient tube formedfrom PTFE that is fitted over the transfer tube 102 and the sensorassembly 100 by expanding it on a mandrel (not shown) or some similarexpanding device and inserting the tube 102 and sensor assembly 100 intothe expanded cover sleeve 120. Thereafter, the cover sleeve 120 isreleased from the expanding device, so that it snugly surrounds the tube102 and sensor assembly 100. The inner surface of the cover sleeve 120is preferably chemically etched to enhance the bond between the sleeve120 and the transfer tube 102, and the cover sleeve 120 is preferablysecured to the transfer tube 102 and the sensor assembly 100 by means ofan epoxy, preferably Master Bond EP-42HT epoxy. Alternatively, the coversleeve 120 may be formed from a heat shrinkable material and may beinstalled by any known method for installed such material.

An alternate, and presently preferred, arrangement of a sensor assemblyis designated generally by reference number 100′ in FIG. 9. The sensorassembly 100′ of FIG. 9 (the cover sleeve 120 (see FIG. 7) is omittedfrom the FIG. 9 for simplicity in the illustration) is similar to thesensor assembly 100 shown in FIG. 7 and previously described, exceptthat the tip element 106 and the tip interface element 114 are replacedby a single tip element 106′ into which the isolating sleeve 112 isinserted as shown. The isolating sleeve 112 is secured to the tipelement 106′ by a suitable epoxy. The exposed section 118 of the ribboncable 66 is attached, preferably by a micro spot weld, directly to thetip element 106′.

In general, the dispense and surface verification system functions asfollows. The transfer tube 102 (FIG. 3) constitutes a first, ortransmitting, electrode for transmitting an oscillating radio frequency(RF) signal that is generated by interface circuitry, as will bedescribed below. The tip element 106 constitutes a second, or receiving,electrode that is electrically isolated from the transfer tube 102(i.e., the first electrode) by means of the isolating sleeve 112. Thetip element 106 functions as a receiver for receiving the signalstransmitted by the transfer tube 102, and the received signals aretransmitted to interface circuitry, as will be described in more detailbelow, by means of the coaxial ribbon cable 66 and the external cable 64(FIG. 5).

When the fluid delivery conduit assembly 52 is neither dispensing afluid nor in contact with a fluid surface, a certain steady state signalwill be received by the tip element 106 and transmitted via the coaxialribbon cable 66 to the interface circuitry 203. When the fluid deliveryprobe 50 is lowered by the robotic substance transfer mechanism 20 intoa container of fluid so that the tip element 106 of the fluid deliveryconduit assembly 52 contacts the surface of the fluid within thecontainer, the receiving characteristics of the tip element 106 willchange, and thus the nature of the received signal (i.e., the amplitudeand/or the phase of the received signal) will also measurably change. Bymonitoring and detecting this change within the interface circuitry,contact with the fluid surface can be detected. When fluid surfacecontact is detected, an appropriate command signal is generated andtransmitted to the motor(s) effecting vertical movement of the fluiddelivery probe 50 to thereby stop further lowering of the probe 50.

The precise detection of the fluid surface and arresting of the verticalmovement of the fluid delivery probe 50 is important for a number ofreasons. One rather obvious reason is that it is desirable to arrestdownward movement of the probe 50 prior to its contact with the bottomof the container, which could cause damage to the probe 50. Anotherreason is that if a significant portion of the end of the fluid deliveryconduit assembly 52 is submerged in a reagent, the outer surface of theconduit assembly 52 will become coated with that reagent. Because thesame robotic substance transfer device 20, and therefore the sameconduit assembly 52, may be used to transfer different reagents fromvarious reagent containers, it is necessary to clean the conduitassembly 52 between reagent transfers, typically by passing de-ionizedwater through the conduit assembly 52. If a significant portion of theoutside of the conduit assembly 52 is coated with reagent, simplypassing water through the conduit assembly 52 will not adequately cleanthe assembly if it is to be submerged into another reagent. Therefore,it is desirable to keep the tip of the conduit assembly 52 at thesurface of the reagent fluid while the fluid is being drawn into theconduit assembly 52. Appropriate movement controls that are well knownin the art may be employed to slightly lower the fluid delivery probe 50while fluid is being drawn, thereby adjusting for the falling fluidsurface within the container and maintaining the tip of the conduitassembly 52 at the fluid surface.

Delivery of fluid by the fluid delivery conduit assembly 52 can bemonitored and verified, in part, by sensing fluid flow through thesensor assembly 100. More particularly, a section 122 of the isolatingsleeve 112 between the distal end 124 of the transfer tube 102 and theproximal end 126 of the tip element 106 defines a measurement section122. When fluid flows through the sensor assembly 100, that is from thetransfer tube 102, through the measurement section 122, and ultimatelythrough the tip element 106, the presence of fluid in the measurementsection 122 between the transfer tube 102 and the tip element 106detectably alters the nature of the signal transmission between thetransfer tube 102 and the tip element 106. Thus, the signal received bythe tip element 106 will be different from the steady state signalreceived by the tip element 106 before or after fluid passes through thesensor assembly 100, as will be described in further detail below.

If the fluid passing through the measurement section 122 is a conductivefluid, i.e., an ionic fluid, primarily the amplitude of the signalreceived by the tip element 106 will change from that of the steadystate signal. On the other hand, if the fluid passing through themeasurement section 122 is non-conductive, i.e., non-ionic, primarilythe phase of the signal received by the tip element 106 will change fromthat of the steady state signal due to a change in the capacitance ofthe sensor assembly 100. In either case, by monitoring and assessing thenature and magnitude of the change in the received signal with theinterface circuitry, as described in more detail below, the flow offluid through the measurement section 122 can be verified, therebyverifying fluid delivery by the fluid delivery probe 52.

Those skilled in the art will appreciate that many fluids will exhibitcharacteristics that are neither completely ionic or non-ionic. That is,fluids may generate both conductive and capacitive reactive effects.

Confirmation of fluid delivery is facilitated by the in-line pressuresensor 40. That is, when both the sensor assembly 100 and the in-linepressure sensor 40 indicate that fluid is passing through the fluiddelivery conduit assembly 52, fluid delivery is confirmed. On the otherhand, if the in-line pressure sensor and the sensor assembly giveinconsistent fluid delivery indications, an error, or fault detection,signal is generated. The specifics of the fault detection algorithm ofthe preferred embodiment will be described in detail below.

Moreover, the specific characteristics of the received tip signal and/orthe pressure signal (i.e., the shapes of the signal profiles) may befluid dependent and can be experimentally determined for each specificfluid. Thus, the signal profiles can be monitored during fluid deliveryor during a tip wash procedure to verify that the proper fluid wasdelivered through the tip.

An alternative configuration for a fluid delivery probe including afluid dispense and fluid surface verification sensor not shown in thedrawing includes a fluid delivery tube with an elongated sensor rodhaving an outside diameter smaller than the inside diameter of the tubeextending through the tube. The sensor rod has two conductive portionslongitudinally spaced from one another and separated from each other bya substantially non-conductive portion. One conductive portion ispreferably located at the distal end of the sensor rod if the sensor isto be used for fluid surface detection, and the other conductive portionis located above the distal conductive portion. The sensor rod may becoterminous with the tube, or the position of its distal end may varywith respect to the distal end of the tube, depending on the desiredposition of the tube with respect to the fluid surface when the fluidsurface is detected. A signal-transmitting circuit, as described below,is electrically coupled to the upstream conductive portion of the sensorrod, and a signal-receiving circuit, as also described below, iselectrically coupled to the distal conductive portion of the sensor rod.A signal, preferably RF, is transmitted from the upstream conductiveportion of the sensor rod, and at least a portion of the transmittedsignal is received by the signal-receiving circuit through the distalconductive portion of the sensor rod. In a like manner as generallydescribed above, and to be described in further detail below, fluiddispense verification and fluid surface detection can be accomplished bymonitoring one or more characteristics of the received signal. That is,the received signal will detectably change when either the distalconductive portion of the sensor rod contacts a fluid surface or whenfluid flows through the tube around the sensor rod between thetransmitting and receiving conductive portions of the sensor rod.

Interface Circuitry

The interface circuitry, discussed in more detail below, provides the“intelligence” for performing the fluid dispense verification andsurface sensing functions described above and discussed in more detailbelow.

FIG. 10 is a high-level, block diagram illustrating the electricalsensing and detection circuitry of the dispense and surface verificationsystem. Microcontroller 201, such as a model MC68HC16Z1 from theMotorola Corporation, is coupled, via the microcontroller's integralanalog to digital converter, to interface circuitry 203, whichinterfaces with the sensor assembly 100 (FIG. 7) on fluid delivery probe50. More particularly, the interface circuitry 203 drives an RF (radiofrequency) excitation signal through transfer tube 102 and to taperedtip 104. The RF excitation signal transmitted by the tip 104 is receivedby the tip element 106, which acts as an antenna receiver. Pressuresensor 40 detects pressure changes created by fluid moving through thetransfer tube 102 of the fluid delivery conduit assembly 52 andtransmits a corresponding pressure signal to the interface circuitry203.

Microcontroller 201 is shown connected to the interface circuitry 203 ofa single fluid delivery probe 50.

FIG. 11 is a detailed block diagram of the interface circuitry 203. Thecircuit elements relating to the transfer tube 102, the tip element 106,and the pressure sensor 40 are generally grouped into element groups211, 212, and 213, respectively.

The excitation signal transmitted through the transfer tube 102 is an RFsignal, such as a signal in the vicinity of 100 KHz, generated by acrystal oscillator and frequency divider, generally indicated at 220,and processed by resonant sine shaper 221 and drive amplifier 222. Thecrystal oscillator/frequency divider 220 serves as the frequency sourcefrom which the transfer tube 102 (i.e., the transmitting electrode)excitation signal is generated. It comprises a crystal oscillator thatoperates at a higher than preferred frequency of 6 MHz, which is dividedby 64 by a CMOS binary counter divider integrated circuit (74HC4060manufactured by, e.g., Texas Instruments) to produce a frequency of near100 KHz (actually 93.75 KHz). The signal output from crystaloscillator/frequency divider 220 is shaped into a sine wave by shaper221 and then amplified by amplifier 222 before being supplied to thetransfer tube 102. Amplifier 222 preferably includes circuitry thatprotects the amplifier from damage due to a short circuit. Suitableshort-circuit protection circuitry would be well known to one ofordinary skill in the art and will not be discussed in detail herein.Crystal oscillators, sine wave shapers, and drive amplifiers are alsowell known in the art and will not be described in additional detail.The integrated divider circuit is a model 74HC4060 circuit, which alsocontains the active circuitry for the crystal oscillator. Such circuitsare available from a number of vendors, such as, Harris Corporation ofMelbourne, Florida and Texas Instruments of Austin, Tex. One appropriateoscillator is manufactured by ECS Inc., International, of Olathe, Kans.as part number ECS-60-32-7. Sine wave shapers may be constructed frompassive circuit components such as resistors, capacitors, and inductors.Drive amplifiers may be constructed using integrated circuit amplifiersavailable from a number of companies, one of which is NationalSemiconductor Corporation of Santa Clara, Calif.

When the fluid delivery probe 50 is in its “home” position (i.e., theposition when fluid delivery probe 50 is at the upper limit of itsmechanical motion in the direction of the Z-axis), the transfer tube 102is grounded through contact with the structural body of the substancetransfer mechanism 20 because substance transfer mechanism 20 acts as agrounding potential. Excitation loss detector circuitry 224 is designedto detect the grounding of the excitation signal and then generate acorresponding home signal, which informs microcontroller 201 that theprobe is in the home position to thereby stop the motor(s) drivingupward Z-axis motion.

Diode clamping is implemented by static discharge protection circuitry225 to protect elements 212 from excessive static discharge. Thus,excessive static electricity that accumulates on the transfer tube 102will not damage the interface circuitry 203. In operation, if chargeaccumulates above a threshold level allowed by static dischargeprotection circuit 225, the diodes in circuit 225 shunt the excesscharge to ground by way of positive and negative analog power supplyrails (not shown). The threshold level is set low enough to protectelements 212 from damage.

Circuit elements 212 interact with tip element 106 via the signaltransmitted from the tip element by the ribbon cable 66 and externalcable 64. Elements 212 include an amplifier 230, a phase difference toDC conversion phase detector 231, a phase filter and scaling circuit232, a precision rectifier 233, an amplitude filter and scaling circuit234, an auto-tune circuit 235, a tuning information data buffer 236,static discharge protection circuitry 237, and a high-low gain selectcircuit 238. The interaction of tip element 106 and circuit elements 212will be described in more detail below.

Tip element 106 acts as an antenna that receives RF signals transmittedfrom tapered tip 104 of the transfer tube 102. Signals received by thetip element 106 are amplified by amplifier circuit 230 before beingsupplied to phase detector 231 and precision rectifier 233. The phasedetector 231 and precision rectifier 233 produce signals indicative ofthe phase change and the amplitude, respectively, of the signal receivedat tip element 106. By monitoring the temporal changes in these signals,microcontroller 201 detects changes caused by the presence or absence offluids passing through the measurement section 122 between the taperedtip 104 and tip element 106 and/or caused by the tip element 106contacting a fluid surface. Conductive fluids (ionic fluids), forexample, when in contact with tapered tip 104 and tip element 106,effectively act as a conductor between the tip element 106 and taperedtip 104, thus increasing the measured amplitude of the signal receivedby the tip element 106. Less conductive fluids, on the other hand, tendto act more as a dielectric, thereby causing the tapered tip 104 and thetip element 106 to behave as electrodes of a capacitor, thus affectingthe phase shift between the signal transmitted by the transfer tube 102and the signal received by the tip element 106.

Phase detector 231 receives both the amplified tip element signal fromthe amplifier circuit 230 and the original transmission signal generatedby sine shaper 221. Phase detector 231 compares the phase of the twosignals and outputs a direct current (DC) signal having an amplitudecorresponding to the phase difference between the two signals. Theresultant signal is sent to microcontroller 201 by phase filter andscaling circuitry 232 after low-pass filtering and scaling to a levelappropriate for transmission via the analog to digital converter 202. Amore detailed description of phase detector 231 is given below withreference to FIG. 12.

Precision rectifier 233 also receives the output of amplifier circuit230 and rectifies the signal so that only the positive portion of thesignal is sent to amplitude filter and scaling circuit 234, which thenlow-pass filters the received signal to perform a DC averaging operationon the signal (i.e., the RF signal is converted to a DC signal ofrepresentative amplitude). This signal may then be scaled to a levelappropriate for transmission to microcontroller 201 via analog todigital converter 202.

As described above, phase difference detector 231 and precisionrectifier 233 operate in tandem to transmit both the phase shift andamplitude of signals received at tip element 106 to microcontroller 201.Microcontroller 201, by monitoring the temporal changes in signalsreceived at tip element 106, discerns changes in the contact state andthe ionic state of fluids in contact with the sensor assembly 100.Typically, the phase difference signal is monitored for fluid surfacedetection, and the amplitude signal is monitored for dispenseverification as will be described in more detail below.

It is desirable to tune the receiver circuit formed by tip element 106,the ribbon cable 66, and coaxial cable 64, both to tune out undesirablecapacitive reactance of ribbon cable 66 and the coaxial cable 64 and toinitially tune the receiver circuit to be near resonance so that thephase shift between the signal transmitted by transfer tube 102 and thesignal received by tip element 106 is small (e.g., about 10% or less andmost preferable from 2-5%) or non-existent. Auto-tune circuit 235, whichincludes an inductor and a series of capacitors that operate as avariable capacitor, perform this tuning function. Typically, tuning isperformed at system initialization (i.e., when the system is firstturned on). Tuning may be performed only when significant components,e.g., probe 50, are replaced.

Tuning the circuit to near resonance is desirable because resonantcircuits generate maximum amplitude signals and the maximum signal phaseshift in response to excitation. Preferably, the circuit is tuned to apoint slightly below resonance (e.g., 2-5% below resonance) inanticipation of the tip element 106 contacting a fluid surface andpushing the circuit towards resonance. Being tuned slightly belowresonance, the receiver circuit operates in an area of its amplitude andphase resonant response curves where the change in amplitude and phaseis monotonic.

Microcontroller 201, via the auto-tune circuit 235, tunes the circuitslightly below resonance by looking at the phase difference output byphase detector 231 during steady state conditions when no fluid is incontact with the sensor assembly 100. When the phase difference is zero,or nearly zero, the circuit is in resonance.

Physically, auto-tune circuit 235 may comprise an inductor (e.g., a 6.8mH inductor) connected in parallel with a series of capacitors that areelectrically inserted or removed from the circuit based on the datalatched into data buffer 236. Microcontroller 201 monitors the phasedifference output from phase filter and scaling circuit 232 andaccordingly adjusts the variable capacitance of auto-tune circuit 235.The capacitance adjustment is performed using any of a number of knownapproximation algorithms (e.g., a binary approximation algorithm).Alternatively, instead of automatically adjusting the capacitance ofauto-tune circuit 235, the circuit may be manually adjusted by selectinga series of manual switches, such as a DIP (dual in-line package)switch. A more detailed description of auto-tune circuit 235 is givenbelow, with reference to FIG. 13.

Static discharge protection circuit 237, in a manner similar to staticdischarge protection circuit 225, protects circuit elements 213 fromexcessive static discharge.

Depending on the type of fluid (e.g., ionic or non-ionic) in contactwith tip element 106 and/or tapered tip 104, the amplitude of thesignals received by circuitry 212 may vary significantly in both surfacesensing and volume verification applications. To effectively interpretsuch a large dynamic signal range, high-low gain select circuit 238,under control of microcontroller 201, dynamically adjusts (i.e., adjustswhenever necessary) the amplification level of amplifier 230. Inoperation, when the signal level received by microcontroller 201 fromamplifier filter and scaling circuit 234 falls below a preset level,microcontroller 201 instructs high-low gain select circuit 238 toincrease the gain of amplifier 230. Conversely, when the signal levelreceived by microcontroller 201 from filter and scaling circuit 234rises to its maximum level, microcontroller 201 instructs high-low gainselect circuit 238 to decrease the gain of amplifier 230. High-low gainselect circuit 238 is preferably implemented using a binary switch(transistor switched resistor) controlled by microcontroller 201 toswitch between the high-gain state or low-gain state of circuit 238.

Pressure sensing circuitry elements 213 interact with microcontroller201 and pressure sensor 40. More particularly, pressure at pressuresensor 40 changes as fluid is accelerated and decelerated through thetube 32 by pump 36. By monitoring changes in gauge pressure as detectedby pressure sensor 40, the dispense and surface verification system candetect the onset of fluid being aspirated and dispensed. As will bedescribed in more detail below, microcontroller 201 uses the informationfrom pressure sensor 40 in combination with information derived from thesignal received by tip element 106 to verify a proper fluid dispense byfluid delivery probe 50 (FIG. 3).

Pressure sensing circuitry elements 213 (FIG. 11) include a voltagereference circuit 240, a buffer 241, a differential amplifier 242, andzero elevation bias circuit 243. Voltage reference circuitry 240generates a reference voltage that is buffered (temporarily stored) bybuffer 241 before being transmitted to pressure sensor 40. The referencevoltage generated by reference voltage circuitry 240 is used tocalibrate the voltage output from the pressure sensor 40 to the desiredoutput voltage range. Buffer 241 sources the reference voltage to sensor40. Signals generated by pressure sensor 40 are amplified bydifferential amplifier 242 to a level appropriate for transmission tomicrocontroller 201 via analog to digital converter 202. The output ofpressure sensor 40 is a function of both the changing fluid pressure influid delivery conduit assembly 52 caused by pump 36 and the quiescentfluid pressure of the fluid in the conduit assembly 52. Zero elevationbias circuit 243 compensates the signal from sensor 40 to set the valuemeasured by differential amplifier 242 when the fluid is in itsquiescent state to a predetermined value (e.g., 55 of a scale of 0 to255).

FIG. 12 is a detailed circuit diagram illustrating an exemplaryembodiment of the phase difference to DC conversion circuit 231. Ingeneral, conversion circuit 231 operates by converting its two inputsignals from sine shaper 221 and amplifier 230 to square waves,logically ANDing the two square waves, and averaging the logically ORedversion of the signals to obtain an average DC value. The DC value isproportional to the phase difference between the two signals.

The signal received by the tip element 106 is passed through resistor901 to comparator 902, which converts the input signal to a square wave.Similarly, the transmitted excitation signal is passed through resistor920 to comparator 921, which converts the input signal to a square wave.The square waves are logically ANDed by resistor 903, and the resultantsignal is then filtered by resistors 904 and 905 and by capacitors 907and 908. Amplifier 909, in conjunction with resistors 910-912,implements an averaging circuit that averages the filtered signal toobtain the output signal 915.

In operation, the voltage of signal 915, when the input signals arein-phase, is half the pull-up voltage (shown as 5 volts), or 2.5 volts.As the phase between the two input signals shifts, the voltage of signal915 varies. For example, for a phase shift of 90 degrees, the outputvoltage is one-quarter of 5 volts (1.25V). For a phase shift of 45degrees, the output voltage is about 1.87 volts.

Appropriate resistance and capacitance values for the constituentresistors and capacitors of circuit 231 are shown in FIG. 12. Suitablecomparators and amplifiers include, for example, models TLC372CD andTL074CD, respectively, available from Texas Instruments Inc., of Dallas,Tex. The resistors and capacitors are standard electronic components.

FIG. 13 is a detailed circuit diagram illustrating an exemplaryembodiment of the tuning portion of auto-tune circuitry 235.

As previously mentioned, microcontroller 201 dynamically tunes auto-tunecircuit 235 by selecting a specific combination of capacitors 1110-1116that generates a desired equivalent capacitance. Preferably, thecapacitance of each of the capacitors 1110-1116 varies from one anotherbased on a factor of a little less than two. For example, theillustrated capacitor values are: 100 pF (pico-Farad) (capacitor 1110),56 pF (capacitor 1111), 33 pF (capacitor 1112), 18 pF (capacitor 1113),10 pF (capacitor 1114), 6 pF (capacitor 1115), and 3 pF (capacitor1116). Microcontroller 201 selects active combinations of thesecapacitors 1110-1116 by selectively activating or deactivating lines1030-1036. Activation of any one of lines 1030-1036 causes associatedtransistors 1020-1026, respectively, to electrically couple or decoupleone of capacitors 1110-1116 in the RF tuning portion of the circuit.Resistors 1010-1016 connect DC power source 1040 to a terminal ofcapacitors 1110-1116, respectively, and act to minimize collector tobase capacitance effects of transistors 1020-1026.

Microcontroller 201, by selectively activating lines 1030-1036, canchange the equivalent capacitance of capacitors 1110-1116 from about 3pF to 200 pF. Alternate capacitive ranges could be implemented bysubstituting different values for capacitors 1110-1116.

Signal Processing and Analysis

The preferred manner in which signals generated by the sensor assembly100 are used to sense a fluid surface and to confirm a proper fluiddispense will now be described.

In a typical aspirate/dispense sequence, the robotic substance transfermechanism 20 moves the fluid delivery probe 50 to a container of fluid(e.g., an assay reagent) that is to be transferred from the container toa reaction receptacle (e.g., a test tube). After the fluid deliveryprobe 50 is positioned above the container, the substance transfermechanism 20 lowers the fluid delivery probe 50 until the tip element106 of the fluid delivery conduit assembly 52 contacts the fluid surfacewithin the container, as sensed by the sensor assembly 100.

As described above, contact with a fluid surface can be sensed bymonitoring the signal received by the tip element 106 and detecting achange in either the amplitude or the phase shift of the received signalthat occurs when the tip element 106 contacts a fluid surface.Preferably, the fluid surface is sensed by monitoring the phase shiftbetween the signal transmitted by the transfer tube 102 and the signalreceived by the tip element 106 and looking for a change in the phaseshift that will occur when the tip element 106 contacts a fluid surface.Monitoring the phase shift is preferred because the change in phaseshift resulting from fluid surface contact will typically be moredrastic than a change in the amplitude of the received signal. Thus, itwill be easier and more accurate to perform surface sensing bymonitoring phase shift than by monitoring change in signal amplitude.

In particular, when there is no fluid in the measurement section 122 ofthe sensor assembly 100, the tapered tip 104 of the transfer tube 102and the tip element 106 are electrically coupled to each other onlythrough a small capacitance arising from mutual physical proximity. Thesignal transmitted by transfer tube 102 will deviate slightly in phasefrom the signal received by the tip element 106, the deviation being dueto slight off-resonance tuning of the resonant receiving arrangementdescribed above. When the tip element 106 is not in contact with a fluidsurface, the interface circuitry is switched to a high gain by thehigh-low gain select circuit 238, and the receiver circuit formed by thetip element 106, the ribbon cable 66, and the external coaxial cable 64is tuned by the microcontroller 201 using the auto tune circuit 235 tonear resonance (i.e., so that the phase shift between the transmittedand received signals deviates slightly from an in-phase condition aspreviously described). When the tip element 106 contacts a fluidsurface, the phase shift signal detected by the phase detector 231changes, deviating more greatly from an in-phase condition than was thecase prior to fluid contact, thereby causing an almost immediate andeasy to detect jump in the phase shift signal. This jump in the phaseshift will indicate contact with a fluid surface.

The phase change is due to stray capacitance to ground of the sensedfluid and its container. When tip element 106 contacts the fluidsurface, the effect is that of adding additional capacitance to groundfrom the tip due to the dielectric properties of the sensed fluid andits capacitive coupling to the metallic structure (i.e., ground). Thus,the resonant frequency of the tuned circuit decreases due to the addedcapacitance, changing both the phase and amplitude of the signal at thetip element.

When sensing very conductive fluids in this manner, the effect is thatof increasing stray capacitance yet more, as the interface surface areabetween the fluid and its (non-conductive) container serves as one plateof a better defined, larger capacitor, with the other plate being thesurrounding metallic (ground) structure. This is true as a container ofvery conductive fluid behaves electrically almost in the manner of asolid metallic block, i.e., it is conductive to the point whereconductivity within the liquid completely overrides dielectric (internalcapacitance) effects.

A change in amplitude arises due to a greater departure from resonancethan is implemented and fixed by the autotuning algorithm. Operation inthis manner is akin to slope detection, known to those skilled in theart, where detection of frequency deviation utilizes skirt slopes ofresonant response curves for conversion of frequency deviation toamplitude deviation.

When contact with the fluid surface is detected, descent of the fluiddelivery probe 50 is arrested, so that the position of the tip of thefluid delivery conduit assembly 52 is maintained at or just below thefluid surface. Next, the pump 36 is activated to draw (i.e., aspirate)an aliquot of fluid from the container and into the fluid deliveryconduit assembly 52. It may be desirable to transfer multiple aliquotsof fluid from the container to multiple reaction receptacles. Thus, morethan one aliquot may be drawn into the fluid delivery conduit assembly52 so that the multiple aliquots can be dispensed into multiple reactionreceptacles without requiring repeated returns to the container for eachaliquot to be dispensed. Depending on the volume of fluid drawn by thepump 36 and the respective volumes of the fluid delivery conduitassembly 52, the rigid tube extension 34, and the flexible tube 32,fluid may be drawn by the pump 36 up into the rigid tube extension 34and the flexible tube 32.

In the preferred manner of practicing the invention, the pump 36 andpart of the fluid conduit defined by the flexible tube 32 and the rigidtube extension 34 are filled with deionized water to function as adrawing, or pumping, fluid when the pump 36 is activated to draw fluidfrom a container into the fluid delivery conduit assembly 52. Deionizedwater is used because, compared to air, it is incompressible andtherefore better suited than air to function as a drawing fluid foraspirating and dispensing precise amounts of fluid. To prevent theaspirated fluid from becoming contaminated by the water in the fluidconduit, an air gap is maintained within the fluid conduit between thedeionized water and the aspirated fluid.

When fluid is drawn by the pump 36 into the fluid delivery probe 50, thepressure sensor 40 will detect a change in gauge pressure when a fluid(e.g., pure liquid, solution, mixture, slurry, suspension, etc.) isaspirated into the fluid delivery probe 50. This measurable change inpressure can be used to confirm that fluid has indeed been aspirated,and certainly, if only air were aspirated, the sensor 40 would be ableto provide an indication of this fact because there would be essentiallyno change in gauge pressure. On the other hand, if a partial orincomplete aspiration occurred, for example, if there were foam at thesurface of the fluid so that some amount of air were aspirated inaddition to the fluid, the sensor 40 may still detect a measurablechange in pressure. This can happen because, when performing a surfacesensing function, the dispense and surface verification system does notnecessarily have the ability to distinguish between foam and fluid.Thus, if the sensor assembly 100 contacts foam at the fluid surface, theresulting phase shift of the signal received by the tip element 106 maybe sufficient to give a positive fluid surface indication, even if theassembly 100 has not actually contacted the fluid surface.

If at least some fluid were aspirated, along with the foam (i.e., acombination of air and fluid), the magnitude of the pressure change maybe large enough to erroneously indicate a proper aspiration. Properaspiration could be verified by monitoring the period of time that thesensor 40 indicates a pressure change that is above a predefinedthreshold indicative of proper fluid aspiration. If the pressure changelasts for an expected period of time within a predefined limit, properaspiration of a sufficient quantity of fluid can be confirmed. If, dueto the partial aspiration of air, the pressure change does not last foran expected period of time, an improper aspiration is indicated, and anerror code would be returned.

In the preferred manner of practicing the present invention, the linepressure measured by the sensor 40 is not monitored during fluidaspiration. Rather, proper fluid aspiration is confirmed indirectly byconfirming proper dispense of the prescribed amount of each aliquot offluid, as will now be described.

After one or more aliquots of fluid have been aspirated, the roboticsubstance transfer mechanism 20 moves the fluid delivery probe 50 to areaction receptacle and positions the fluid delivery conduit assembly 52for dispensing fluid into the reaction receptacle. The accuracy andintegrity of results obtained from tests performed in the reactionreceptacle(s) are dependent on, among other factors, dispensing theproper amount of each assay reagent into the receptacle(s). In otherapplications involving the fluid dispense and fluid surface verificationdevice and method of the present invention, the accuracy of test resultsmay not be at stake, but verification of proper fluid dispense may,nonetheless, be important. Regardless of the application, the presentinvention provides an apparatus and method for accurately verifying aproper dispense of fluid.

During fluid dispense, the pump 36 is activated for a discrete period inorder to force a discrete amount of fluid through the fluid deliveryconduit assembly 52 and into an awaiting receptacle. Movement of fluidthrough the conduit assembly 52 under the force of the pump 36 willcause a measurable increase in the fluid pressure, as sensed by thepressure sensor 40. Similarly, movement of fluid through the measurementsection 122 of the sensor assembly 100 will cause a measurable change inthe amplitude and/or the phase of the signal received by the tip element106.

Furthermore, the fluid dispense verification capability of the system ispreferably used to verify the passage of a cleansing fluid, such asdeionized water, through the probe assembly 52 in response to the actionof a pump constructed and arranged to move such cleansing fluid throughthe assembly 52.

FIG. 14 shows exemplary pressure sensor and tip element signalssuperimposed on a dimensionless amplitude (analog to digital, or “A/D”,counts) versus time (discrete data samples @ 2 msec intervals) plot fora normal dispense sequence of a particular fluid. A travel gap isemployed in the dispense sequence represented in the plots of FIG. 14. Atravel gap is a pocket of air that is drawn into the conduit assembly 52through the tip element 106 and resides between the distal end of thetip element 106 and the bottom surface of a fluid previously drawn intoand contained within the assembly 52. The purpose of the travel gap isto prevent hanging drops of fluid from dislodging when the probe 50 isbeing moved from a fluid container to a reaction receptacle. While thesize of the air gap is not critical it should be of sufficient volume toprevent the release of any fluid from the tip element 106 when the probe50 is in transit.

Before the pump 36 is activated to dispense fluid, both the pressuresignal and the tip signal exhibit a steady quiescent state, generallyindicated by the portions A and H, respectively, of the pressure signaland the tip signal shown in FIG. 14. When the pump 36 is first activatedto dispense, the pressure signal exhibits an increase at an inflectionpoint indicated at B. The pressure signal exhibits a positive slope asthe pump accelerates toward its final velocity. It has been noted duringexperiments that the pressure signal will exhibit an interruption,generally indicated at C, in the positive slope during pumpacceleration. It is believed that this is due to the fact that duringinitial pump acceleration, the travel gap is being forced out of thefluid delivery conduit assembly 52, and, due to the compressibility ofthe air in the travel gap, the pressure signal slope decreases brieflyuntil the travel gap is forced out of the conduit assembly 52. In fact,in dispensing experiments in which there is no travel gap in the fluiddelivery conduit assembly, it has been noted that the pressure signaldoes not exhibit this interruption during pump acceleration.

After the interruption C, the pressure signal exhibits a substantiallyconstant positive slope, indicated at D, that is directly related to theacceleration of the pump. When the pump reaches and maintains itsmaximum velocity, the pressure signal levels off as indicated at E. Thepump is operated at its maximum velocity for a prescribed period of timeto dispense an aliquot of fluid and is then stopped. When the pumpstops, the pressure in the system conduit, and thus the pressure signal,drops almost instantaneously, as shown at F, back toward its quiescentlevel. Shortly after dispensing is terminated by stopping the pump, iffluid remains in the conduit assembly 52, the pump is activated in areverse direction to generate a drop in system pressure, as shown at G,to thereby draw a travel air gap into the conduit assembly 52 beforemoving the fluid delivery probe 50 to the next receptacle that is toreceive an aliquot of fluid.

The tip signal, which is the amplitude of the signal received by the tipelement 106 of the sensor assembly 100, is an indication of when thereis a conductive path through the measurement section 122 connecting thedistal end 124 of the transfer tube 102 and the proximal end 126 of thetip element 106. For non-conductive fluids a similar signal of phaseshift vs time would be analyzed.

In the embodiment of the sensor assembly 100′ shown in FIG. 9, themeasurement section 122 is defined between the distal end 124 of thetapered tip 104 of the transfer tube 102 and an exposed section 108′ ofthe tip element 106′ at the end of the isolating sleeve 112. Otherwise,the sensor assembly 100′ operates similarly to the sensor assembly 100in the sense that the tapered tip 104 functions as a signal transmittingelectrode and the tip element 106′ functions as a signal receivingelectrode that is electrically isolated from the tapered tip 104. Onebenefit of the sensor assembly 100′ shown in FIG. 9 over the sensorassembly 100 shown in FIG. 7 is that the proximal end 126′ of the tipelement 106′ of the assembly 100′ is outside the fluid flow path. On theother hand, the proximal end 126 of the tip element 106 of the assembly100 is inside the fluid flow path and thus forms a surface where fluidbuildup can potentially occur.

As shown in FIG. 14, the tip signal remains substantially at itsquiescent level, indicated at H, for a brief period after the pressuresignal has started rising. Due to the travel air gap, there is a briefperiod after the pump is activated during which the measurement section122 is not full of fluid, so there is no conductive connection betweenthe transfer tube 102 and the tip element 106. After the travel gap hasbeen forced through the measurement section 122, the tip signalamplitude jumps almost instantaneously, as shown at I, to its maximumlevel indicating conduction (i.e., a short) between the transfer tube102 and the tip element 106. The tip signal amplitude will exhibit thissteady state level, as shown at J, as long as there is a conductivefluid in the measurement section 122.

In fact, in a proper dispense, where there are multiple aliquots to bedispensed, the tip signal amplitude will maintain this level for aperiod after the pump stops, as shown at L after the tip signal hasintersected the pressure signal, until a travel air gap is drawn intothe conduit assembly 52 to break the conduction between the transfertube 102 and the tip element 106 to thereby cause the tip signalamplitude to drop almost instantaneously, as shown at M.

It has been empirically determined by monitoring abnormal dispensescreated by simulating system malfunctions, such as fluid foaming, loosefluid conduit fittings, and low system fluid level, that abnormaldispenses can be detected by monitoring and evaluating four features ofthe pressure and tip signals: 1) the pressure pulse width (P_(PW)); 2)the pressure signal integral (P_(int)); 3) the tip signal amplitudevariability; and 4) the tip signal amplitude integral.

The pressure pulse width (P_(PW)) is the width (along the time axis) ofthe pressure signal from the beginning of the pressure pulse rise(P_(start)), point B, to the sharp fall when the pump stops (P_(stop)),point F. Ideally, to find P_(start), a window is set around the expectedpressure signal transition and the data points in the window areevaluated and compared to a threshold value to determine if thetransition occurs. Preferably, the dispense and surface verificationsystem is in communication with the pump so the system will “know” whento expect a transition in the pressure signal based on activation of thepump. A threshold value may be defined by averaging a suitable number(e.g., 16) of data points taken during the quiescent portion of thepressure data before the pump has been activated and adding a prescribednumber (e.g. 20) to the quiescent average. For example, if the averagevalue of the pressure data during the quiescent portion of the signalwere 40 A/D counts, the threshold value may be set at 60 A/D counts.When the pressure data exceeds the predefined threshold, a pressuretransition is indicated and P_(start) is located.

Similarly, P_(stop) may be defined at the point where the pressure valuefalls below the threshold level or some other predefined percentage ofthe maximum pressure, for example 50% of the maximum pressure value.

Another method for finding P_(start) and/or P_(stop) would be to performa slope detection function on sliding groups of data points nearexpected pressure transitions until a sharp change in the slope isdetected. For example, P_(stop) can be found by centering a window ofsuitable width at a point spaced from P_(start) by the anticipated pulsewidth and searching for a radical downward transition (i.e., a slopechange) in the pressure signal. If the transition is found, recordP_(stop) at the beginning of the transition. If no transition is found,an error code is returned.

Assuming that P_(stop) and P_(start) are found, the pulse width,P_(stop)−P_(start), is compared to experimentally-determined low andhigh limits of the pulse width designated P_(PWLO) and P_(PWHI),respectively. The limits P_(PWHI) and P_(PWLO) are unique to eachreagent that may be transferred with the fluid delivery probe 50 and canbe downloaded into or previously stored in the dispense and surfaceverification diagnostic software.

If P_(PW) is within the expected limits, the pressure signal isintegrated (P_(int)) from P_(start) to P_(stop). That is, the area underthe pressure signal curve between P_(start) and P_(stop) is computed.P_(int) is defined as the sum of all of the discrete data pressurepoints during pump operation. More particularly, P_(int) is determinedby subtracting the base line area under the curve from the integralcalculated from P_(start) to P_(stop). The base line area under thecurve, i.e., the baseline integral, is obtained by multiplying theaverage baseline pressure signal value (before pumping started) by thederived pulse width, P_(PW). Experimentally-determined limits P_(intLO)and P_(intHI), which are also unique for each reagent, are downloadedinto or stored in the dispense and surface verification diagnosticsoftware, and the calculated P_(int) is evaluated to determine whetherit is within these limits. If P_(int) is within the expected limits,processing may continue; if not, an error code is returned.

Normally the integral of a pressure versus time signal (i.e., the areaunder the pressure-time signal) would be equal to the volume of fluiddispensed during pump movement. In the preferred application of thedispense and surface verification system of the present invention,however, the pressure and tip signals are recorded merely asdimensionless A/D counts to provide indications of relative changes inthe respective signals, without indicating the actual magnitudes of therespective signals. A dispense and surface verification system may bemodified, however, by providing system calibration so that pressuresignal voltage is converted to actual pressure magnitude. Thus, thepressure signal integral, calculated as described above, would providethe volume of fluid dispensed during pump movement.

The tip signal integral is designated T_(int) and is defined as the sumof the tip amplitude signal data points starting at the risingtransition of the tip signal, section I, designated T_(start), andending at P_(stop). In other words, the integral is calculated for thetime during which fluid is actually flowing through the measurementsection 122. T_(start) can be determined by monitoring the tip signalamplitude and designating T_(start) as that point where the tip signaldata exceeds a predefined threshold, as described above with respect toP_(start). Alternatively, T_(start) can be located by performing a slopedetection function on the tip signal data and locating a sharptransition (i.e., jump in slope). As with the pressure integral P_(int),the tip signal integral T_(int) can be determined by simple integration.

T_(int) is calculated from T_(start) to P_(stop) and is compared againstexperimentally-determined limits T_(intLO) and T_(intHI), which areunique to each reagent. If T_(int) is not within the expected limits, anerror code is returned.

An irregularity in the tip signal, which is indicative of adiscontinuity in fluid flow between the tapered tip 104 and the tipelement 106 (due to, e.g., pump malfunction, probe blockage, air bubblesin the dispensed fluid, insufficient fluid available for dispensing),will result in a value of T_(int) that is not within expected limits. Onthe other hand, a value of T_(int) that is within expected limits isindicative of a regular tip signal and thus a proper fluid dispense.

If no travel air gap is employed, fluid fully fills the measurementsection 122 prior to pumping, so there will be no transition in the tipsignal amplitude. Thus, T_(start) cannot be determined by comparing tipsignal data to a threshold value or by preforming a slope detection. Thestarting point, T_(start) for determining T_(int), can be defined sometime after P_(start) by moving out a predetermined number of datasamples from P_(start). The number of samples can be determinedexperimentally from typical data (it will be reagent-specific) andrepresents the time before fluid would have reached the measurementsection 122 if there had been a travel gap. Ideally, the starting point,T_(start), selected should correspond to the beginning of a fluiddispense.

The tip signal amplitude variability is indicated by T_(hcv) (derivedfrom coefficient of variance of the horizontal tip signal). During anormal dispense, once fluid fills the measurement section 122 of thesensor assembly 100 during pump acceleration, the tip signal should besubstantially constant through the end of pump movement or P_(stop), asdemonstrated by section J of the tip signal of FIG. 14. If the tipsignal is not substantially constant, this is an indication that fluidflow through the measurement section 122 is not constant, a conditionthat can occur if air bubbles are aspirated into the system. Forexample, see FIG. 15, which shows exemplary pressure signals for fluiddispenses in which various amounts of air are trapped in the fluid. Airbubbles being aspirated into the system often result from a faultysurface sense prior to fluid aspiration, where aspiration is commencedwhen the tip of the probe assembly 52 is slightly above the fluidsurface.

T_(hcv) is determined by evaluating the tip signal data points startingjust beyond the rising transition, where the tip signal integralsummation is started, and continuing until P_(stop). The standarddeviation of the points divided by the mean of all the data pointsresults in T_(hcv), and is expressed as a percent. For each reagent, amaximum tip signal variability T_(hcvMax) is determined experimentally,and the calculated T_(hcv) is compared to this maximum.

If T_(hcv) is above an expected T_(hcvMax), an error code is returned.The variability that can be tolerated will depend on the particularapplication.

While the invention has been described in connection with what arepresently considered to be the most practical and preferred embodiments,it is to be understood that the invention is not to be limited to thedisclosed embodiments, but, on the contrary, is intended to covervarious modifications and equivalent arrangements included within thespirit and scope of the appended claims.

Furthermore, those of the appended claims which do not include languagein the “means for performing a specified function” format permittedunder 35 U.S.C. §112(¶6), are not intended to be interpreted under 35U.S.C. §112(¶6) as being limited to the structure, material, or actsdescribed in the present specification and their equivalents.

1. A method of detecting a fluid surface comprising: transmitting asignal with a signal transmitting device; receiving at least a portionof the signal with a signal receiving device; detecting a change inamplitude of the received signal as an indication that the signalreceiving device has contacted the fluid surface; and minimizing theamplitude of the received signal prior to contacting the fluid surfacewith the signal receiving device.
 2. The method of claim 1, furthercomprising grounding the signal transmitting device when the signalreceiving device is in a predetermined position not in contact with thefluid surface.
 3. The method of claim 1, further comprising amplifyingthe received signal prior to detecting the changes in the amplitude ofthe received signal after contacting the fluid surface with the signalreceiving device.
 4. The method of claim 3, wherein the step ofamplifying the received signal further comprises amplifying theamplitude of the received signal to substantially the same amplitude asthe transmitted signal.
 5. The method of claim 3, wherein the step ofamplifying the received signal further comprises dynamically adjustingthe amplification level according to the measured amplitude of thereceived signal.
 6. The method of claim 5, further comprising amplifyingthe received signal to either a high or low gain level to increase themeasured amplitude of the received signal to a desired level.
 7. Themethod of claim 6, wherein the low gain level amplification correspondsto contact of the signal receiving device with a conductive fluid. 8.The method of claim 6, wherein the high gain level amplificationcorresponds to contact of the signal receiving device with anon-conductive fluid.
 9. The method of claim 1, further comprisingamplifying the received signal prior to detecting the changes in theamplitude of the received signal after contacting the fluid surface withthe signal receiving device and comparing the minimized received signalto the amplified received signal.
 10. The method of claim 1, furthercomprising tuning the received signal so that the relationship betweenchanges caused by contacting the fluid surface and changes in anamplitude of the received signal is monotonic.
 11. A method of detectinga fluid surface comprising: transmitting an RF signal with a signaltransmitting device; receiving at least a portion of the signal with asignal receiving device; and detecting a change in at least onecharacteristic of the received signal as an indication that the signalreceiving device has contacted the fluid surface.
 12. The method ofdetecting a fluid surface of claim 11, further comprising moving thesignal receiving device into contact with the fluid surface.
 13. Themethod of detecting a fluid surface of claim 11, wherein the signalreceiving device is mobile.
 14. A method of detecting a fluid surfacecomprising: transmitting a signal with a signal transmitting device;receiving at least a portion of the signal with a signal receivingdevice; and detecting changes in a phase shift between the transmittedand received signals as an indication that the signal receiving devicehas contacted the fluid surface.
 15. The method of claim 14, furthercomprising tuning the received signal so that the transmitted andreceived signals are near resonance prior to the signal receiving devicecontacting the fluid surface.
 16. The method of claim 14, furthercomprising tuning the received signal so that the phase differencebetween the transmitted and received signals is small prior to thesignal receiving device contacting the fluid surface.
 17. The method ofclaim 16, further comprising tuning the received signal so that there isabout a 2-5% phase difference between the transmitted and receivedsignals prior to the signal receiving device contacting the fluidsurface.
 18. The method of claim 14, further comprising tuning thereceived signal so that the relationship between contacting the fluidsurface and phase shift between the transmitted and received signals ismonotonic.
 19. A method of detecting a fluid surface comprising:transmitting a signal with a signal transmitting device; receiving atleast a portion of the signal with a signal receiving device; detectinga change in at least one characteristic of the received signal as anindication that the signal receiving device has contacted the fluidsurface; and tuning a receiver circuit operatively coupled to the signalreceiving device to be near resonance prior to the signal receivingdevice contacting the fluid surface, wherein said tuning step comprisestuning the receiver circuit so as to be about 2-5% below resonance priorto the signal receiving device contacting the fluid surface.